1. Field of the Invention
The present invention relates to an optical scanning device and an image forming apparatus including the same.
2. Description of the Related Art
With improvement in colorization and high-speed processing of an electrophotographic image forming apparatus such as a laser printer, a laser plotter, a digital copier, a facsimile machine, and a multifunction product, there has been widely used a tandem-type image forming apparatus including a plurality of (typically four) photosensitive elements as image carriers. For example, in the tandem-type image forming apparatus, four photosensitive elements are arranged along a transfer belt (or intermediate transfer belt) that conveys a recording material. A charging unit charges the photosensitive elements, and then a writing unit forms latent images on the photosensitive elements, one on each of them. A developing unit then develops the latent images using developers of different colors (for example, toner of yellow, magenta, cyan, and black) into visible images. The visible images of different colors are superimposed and transferred onto the recording material conveyed by the transfer belt (or the intermediate transfer belt), so that a color image is formed.
Some electrophotographic color image forming apparatuses that employ a single-drum intermediate transfer system includes only one photosensitive element. In the single-drum intermediate transfer system, a photosensitive element is rotated the number of colors, e.g., four times, per color image, so that images of the colors are sequentially transferred onto an intermediate transfer member as superimposed images. Afterward, the superimposed images are collectively transferred onto a recording material. In such an image forming apparatus, however, it is necessary to rotate the photosensitive element four times for forming a full-color image, which causes lower productivity than the tandem-type image forming apparatus.
Thus, the tandem-type image forming apparatus can achieve a higher processing speed and a higher productivity than the single-drum image forming apparatus. However, in the tandem-type image forming apparatus, a writing unit performs optical writing on a plurality of photosensitive elements by using an optical scanning unit including a light source corresponding to each photosensitive element. For example, four light sources are usually necessary for writing to four photosensitive elements. An increase of the light sources causes an increase of the number of components to be used, color shift due to a wavelength difference between the light sources, and an increase in costs.
Furthermore, deterioration of a laser diode is one of the reasons for malfunction of the writing unit. Therefore, the increase of the light sources causes a higher probability of malfunction of the light source, resulting in a decrease of recyclability.
A conventional technology for decreasing the number of light sources used in the optical scanning device is disclosed in Japanese Patent Application Laid-open No. 2002-23085. A pyramidal mirror or a flat-plate mirror is used to scan a plurality of surfaces to be scanned with light beams emitted from a common light source. Although the conventional technology can reduce the number of light sources, a light beam needs to be deflected by two surfaces of deflecting mirrors, which makes it difficult to perform high-speed processing.
Japanese Patent Application Laid-open No. 2006-284822 discloses a conventional technology for solving the above-describe problem. In an optical scanning device according to the conventional technology, light beams emitted from a light source is split into two light fluxes by a splitting unit such that the two light fluxes is displaced to each other in a sub-scanning direction. The two light fluxes enter a deflecting unit including two polygon mirrors that are aligned at different angles and concentrically rotate. After deflected by the deflecting unit, the two light fluxes are projected onto two different surfaces to be scanned, respectively.
The conventional technology uses the two stages of polygon mirrors for generating phase shift, so that it is possible to scan different surfaces with light beams from a common light source. However, such a polygon mirror is not used for general purpose, thereby causing an increase of costs. Furthermore, high proccessability of the polygon mirror is required. Because surface deviation or surface accuracy of each polygon mirror is different, image quality can be decreased.